One of the considerable benefits of an automatic transmission is the ability to creep forward in heavy traffic by simply releasing the foot brake. In contrast, a manual transmission vehicle has to be eased gently forward under control from the accelerator, brake and clutch pedals, which quickly becomes tiresome for the driver and is not efficient from the perspective of the fuel economy.
The issue of fuel economy under such conditions has been addressed by the introduction of engine start-stop technology which typically requires the transmission to be in neutral for the engine to stop in vehicles having a manual transmission. As a result, in a manual transmission vehicle with start-stop technology, the user not only has to control forward motion with accelerator, brake and clutch, but also has to shift to neutral when the vehicle stops. The repeated shifting from neutral to first gear, the repeated stopping and starting of the engine and the requirement to operate accelerator, brake and clutch combine to create an unsatisfactory experience for the driver.
In addition, hybrid electric vehicles (HEVs) typically have multiple power sources, i.e. an internal combustion engine and an electric motor. The role of the electric motor differs depending on the type of hybrid vehicle.